how to symetrize a part of the mesh in its normal orientation/position in edit mode?
In general:


Establish your Pivot (transform origin, somewhere in the mirror plane). This probably involves ShiftS snapping the 3D Cursor to a known point, or the median of some selection, and setting the 'Pivot' to 'Cursor'.
Establish your Transform Orientation, either using the View, or more likely a Custom Orientation (the '+' at the right of the Transform Orientation dropdown). For example, it could be the normal of a face: select the face, and create an orientation from it. Z in that orientation will now be the direction of the face's normal.
ShiftD duplicate your part..
SZ-1 scale the duplicate by -1 in the appropriate axis of your custom orientation, about your pivot.


You may have to ShiftN, turn the mirrored duplicate right-side-out.